In order for T cells to respond to foreign polypeptides, two signals must be provided by antigen-presenting cells (APCs) to resting T lymphocytes (Jenkins, M. and Schwartz, R. (1987) J. Exp. Med. 165:302-319; Mueller, D. L. et al. (1990) J. Immunol. 144:3701-3709). The first signal, which confers specificity to the immune response, is transduced via the T cell receptor (TCR) following recognition of foreign antigenic peptide presented in the context of the major histocompatibility complex (MHC). The second signal, termed costimulation, induces T cells to proliferate and become functional (Lenschow et al. (1996) Annu. Rev. Immunol. 14:233). Costimulation is neither antigen-specific, nor MHC-restricted, and is thought to be provided by one or more distinct cell surface molecules expressed by APCs (Jenkins, M. K. et al. (1988) J. Immunol. 140:3324-3330; Linsley, P.S. et al. (1991) J. Exp. Med. 173:721-730; Gimmi, C.D. et al. (1991) Proc. Natl. Acad Sci. USA 88:6575-6579; Young, J. W. et al. (1992) J. Clin. Invest. 90:229-237; Koulova, L. et al. (1991) J. Exp. Med 173:759-762; Reiser, H. et al. (1992) Proc. Natl. Acad Sci. USA 89:271-275; van-Seventer, G. A. et al. (1990) J. Immunol. 144:4579-4586; LaSalle, J. M. et al. (1991) J. Immunol. 147:774-80; Dustin, M. I. et al. (1989) J. Exp. Med. 169:503; Armitage, R. J. et al. (1992) Nature 357:80-82; Liu, Y. et al. (1992) J. Exp. Med. 175:437-445).
The CD80 (B7-1) and CD86 (B7-2) proteins, expressed on APCs, are critical costimulatory molecules (Freeman et al. (1991) J. Exp. Med. 174:625; Freeman et al. (1989) J. Immunol. 143:2714; Azuma et al. (1993) Nature 366:76; Freeman et al. (1993) Science 262:909). B7-2 appears to play a predominant role during primary immune responses, while B7-1, which is upregulated later in the course of an immune response, may be important in prolonging primary T cell responses or costimulating secondary T cell responses (Bluestone (1995) Immunity 2:555).
One ligand to which B7-1 and B7-2 bind, CD28, is constitutively expressed on resting T cells and increases in expression after activation. After signaling through the T cell receptor, ligation of CD28 and transduction of a costimulatory signal induces T cells to proliferate and secrete IL-2 (Linsley, P. S. et al. (1991) J. Exp. Med. 173:721-730; Gimmi, C. D. et al. (1991) Proc. Natl. Acad. Sci. USA 88:6575-6579; June, C. H. et al. (1990) Immunol. Today 11:211-6; Harding, F. A. et al. (1992) Nature 356:607-609). A second ligand, termed CTLA4 (CD152) is homologous to CD28 but is not expressed on resting T cells and appears following T cell activation (Brunet, J. F. et al. (1987) Nature 328:267-270). CTLA4 appears to be critical in negative regulation of T cell responses (Waterhouse et al. (1995) Science 270:985). Blockade of CTLA4 has been found to remove inhibitory signals, while aggregation of CTLA4 has been found to provide inhibitory signals that downregulate T cell responses (Allison and Krummel (1995) Science 270:932). The B7 molecules have a higher affinity for CTLA4 than for CD28 (Linsley, P. S. et al. (1991) J. Exp. Med. 174:561-569) and B7-1 and B7-2 have been found to bind to distinct regions of the CTLA4 molecule and have different kinetics of binding to CTLA4 (Linsley et al. (1994) Immunity 1:793). A new molecule related to CD28 and CTLA4, ICOS, has been identified (Hutloff et al. (1999) Nature 397:263; WO 98/38216), as has its ligand, which is a new B7 family member (Aicher A. et al. (2000) J. Immunol. 164:4689-96; Mages H. W. et al. (2000) Eur. J Immunol. 30:1040-7; Brodie D. et al. (2000) Curr. Biol. 10:333-6; Ling V. et al. (2000) J. Immunol. 164:1653-7; Yoshinaga S. K. et al. (1999) Nature 402:827-32). If T cells are only stimulated through the T cell receptor, without receiving an additional costimulatory signal, they become nonresponsive, anergic, or die, resulting in downmodulation of the immune response.
Immune cells have receptors that transmit activating signals. For example, T cells have T cell receptors and the CD3 complex, B cells have B cell receptors, and myeloid cells have Fc receptors. In addition, immune cells bear receptors that transmit signals that provide costimulatory signals or receptors that transmit signals that inhibit receptor-mediated signaling. For example, CD28 transmits a costimulatory signal to T cells. After ligation of the T cell receptor, ligation of CD28 results in a costimulatory signal characterized by, e.g., upregulation of IL-2rα, IL-2rβ, and IL-2rγ receptor, increased transcription of IL-2 messenger RNA, and increased expression of cytokine genes (including IL-2, IFN-γ, GM-CSF, and TNF-α). Transmission of a costimulatory signal allows the cell to progress through the cell cycle and, thus, increases T cell proliferation (Greenfield et al. (1998) Crit. Rev. Immunol. 18:389). Binding of a receptor on a T cell which transmits a costimulatory signal to the cell (e.g., ligation of a costimulatory receptor that leads to cytokine secretion and/or proliferation of the T cell) by a costimulatory ligand results in costimulation. Thus, inhibition of an interaction between a costimulatory ligand and a receptor that transmits a costimulatory signal on immune cells results in a downmodulation of the immune response and/or specific unresponsiveness, termed immune cell anergy. Inhibition of this interaction can be accomplished using, e.g., anti-CD28 Fab fragments, antibodies to B7 family molecules, or by using a soluble form of a receptor to which a B7 family member molecule can bind as a competitive inhibitor (e.g., CTLA4Ig).
Inhibitory receptors that bind to costimulatory molecules have also been identified on immune cells. Activation of CTLA4, for example, transmits a negative signal to a T cell. Engagement of CTLA4 inhibits IL-2 production and can induce cell cycle arrest (Krummel and Allison (1996) J. Exp. Med. 183:2533). In addition, mice that lack CTLA4 develop lymphoproliferative disease (Tivol et al. (1995) Immunity 3:541; Waterhouse et al. (1995) Science 270:985). The blockade of CTLA4 with antibodies may remove an inhibitory signal, whereas aggregation of CTLA4 with antibody transmits an inhibitory signal. Therefore, depending upon the receptor to which a costimulatory molecule binds (i.e., a costimulatory receptor such as CD28 or an inhibitory receptor such as CTLA4), B7 molecules including B7-4 can promote T cell costimulation or inhibition.
PD-1 is a member of the immunoglobulin family of molecules (Ishida et al. (1992) EMBO J. 11:3887; Shinohara et al. (1994) Genomics 23:704). PD-1 was previously identified using a subtraction cloning based approach designed to identify modulators of programmed cell death (Ishida et al. (1992) EMBO J. 11:3887-95; Woronicz et al. (1995) Curr. Top. Microbiol. Immunol. 200:137). PD-1 is believed to play a role in regulating lymphocyte survival, e.g., during clonal selection (Honjo (1992) Science 258:591; Agata et al. (1996) Int. Immunology 8:765; Nishimura et al. (1996) Int. Immunology 8:773). PD-1 has an extracellular region containing an immunoglobulin superfamily domain, a transmembrane domain, and an intracellular region which includes an immunoreceptor tyrosine kinase-based inhibitory motif (ITIM) (Ishida et al. (1992) supra; Shinohara et al. (1994) supra; U.S. Pat. No. 5,698,520). This features also define a larger family of molecules, called the immunoinhibitory receptors, which also includes gp49B, PIR-B, and the killer inhibitory receptors (KIRs) (Vivier and Daeron (1997) Immunology Today 18:286). It is often assumed that the tyrosyl phosphorylated ITIM motif of these receptors interacts with the SH2-domain-containing phosphatases, which leads to inhibitory signals. A subset of these immunoinhibitory receptors binds to MHC molecules, for example the KIRs, and CTLA4 binds to B7-1 and B7-2. It has been proposed that there is a phylogenetic relationship between the MHC and B7 genes (Henry et al. (1999) Immunology Today 20:285-288).
PD-1 was also implicated as a regulator of B cell responses (Nishimura (1998) Int. Immunology 10:1563). Unlike CTLA4, which is found only on T cells, PD-1 is also found on B cells (in response anti-IgM) and on a subset of thymocytes and myeloid cells (Agata et al. (1996) supra; Nishimura et al. (1996) Int. Immunology 8:773).
The importance of the B7:CD28/CTLA4 costimulatory pathway has been demonstrated in vitro and in several in vivo model systems. Blockade of this costimulatory pathway results in the development of antigen-specific tolerance in murine and human systems (Harding, F. A. et al. (1992) Nature 356:607-609; Lenschow, D. J. et al. (1992) Science 257:789-792; Turka, L. A. et al. (1992) Proc. Natl. Acad. Sci. USA 89:11102-11105; Gimmi, C. D. et al. (1993) Proc. Natl. Acad. Sci. USA 90:6586-6590; Boussiotis, V. et al. (1993) J. Exp. Med 178:1753-1763). Conversely, expression of B7 by B7-negative murine tumor cells induces T-cell mediated specific immunity accompanied by tumor rejection and long lasting protection to tumor challenge (Chen, L. et al. (1992) Cell 71:1093-1102; Townsend, S. E. and Allison, J. P. (1993) Science 259:368-370; Baskar, S. et al. (1993) Proc. Natl. Acad. Sci 90:5687-5690.). Therefore, manipulation of the costimulatory pathways offers great potential to stimulate or suppress immune responses in humans.